There are two issues here;
1. The Intellectual Property Rights (IPR) pertaining to the Code.
2. The export of a US Defense Article ie. the code.
1. If the IPR is not a shared asset (eg owned by Lockheed or a sub-contractor) its release and distribution is a matter of commercial agreement (or not in this case) and is nothing to do with vacuous politicians in monkey suits making promises they can't keep. The code is someone's property and earnings for the next 30 years - if you want it, you buy it, it's business.
2. If the code is deemed to contain material that the US regards as a Defense Article (what are the chances?) then any subsequent release to a Foreign Person must be controlled by an export license. If the US State Department decides it can't be exported, it can't be exported, business or not (see case 1). Sure the State Department can be "advised" about export release, but only if the US really wants to go there.
This is a really unique and new experience so you can see why everyone is totally surprised. This hasn't ever happened before, well apart from all the other times it has happened. Ask someone how much source code the MoD owns for the US bits of the Apache - it's a round number.
If access to the source was made a contractual liability it would be there. The fact that it isn't should tell you all you need to know.
PS. Good to see the Static Code Analysis flat-earthers are alive and well in the MoD.
Old news is so exciting.